THEY have dwelt in Scotland’s waterways since the last Ice Age and were, according to Roman historians, the reason why Julius Caesar invaded Britain.
But the freshwater pearl mussel, already critically endangered, is facing a potentially terminal threat from another foreign invader.
American signal crayfish released into the wild have spread through the nation’s rivers in recent decades and now directly threaten the remaining colonies of the rare molluscs.
Scottish Natural Heritage (SNH), the government’s countryside protection agency, says crayfish are now just 20 miles away from a prime colony of freshwater pearl mussels in the Tay.
Anglers are being encouraged to protect the mussel beds from being destroyed. The Tay Fisheries Board is urging fishermen and other river users to avoid anything which could help spread the crayfish, such as eggs which have attached to fishing gear. They also stress the need to kill any adult crayfish found and never return them to the water.
The warning follows SNH-supervised experiments which have demonstrated that crayfish will attack colonies when they eventually invade mussel habitat.
Scottish Natural Heritage’s freshwater adviser, Dr Colin Bean, said they now had evidence of a “mortal” threat to the mussel beds.
“Upstream in the River Earn, there are crayfish around Comrie,” he said. “You get mussels as far down as Perth. They haven’t clashed yet on the Tay, but there is a threat given how fast they spread. It’s not far away.”
Scotland is home to half the world’s population of freshwater pearl mussels. They have been harvested close to extinction on the off-chance they might contain a pearl, and are sensitive to pollution.
The precise location of surviving colonies is publicised as little as possible to give the mussel numbers a chance to recover.
But Bean said that after identifying the risk, SNH wanted to look into what would happen when the creatures met.
He said: “We put the two of them in tanks, looking at the behaviour of the crayfish.
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